Pneumatic force balance relay



July 30, 1957 c. J. swARTWoUT ET AL PNEUMATIC FORCE BALANCE RELAY 2Sheets-SkaaiI 1 Filed Sept. 6. 1952 N fn 1NVE`NToR5 CHARLES J. SWARTWOUTEARL O. SCHWEITZER NYS l, July-30, 1957 Y 2 Sheets-Shea?l 2 File@ sept.e, 1952 INVENTORS. CHARLES J. SWARTWOUT EARL O. SCHWUTZER ATTORNEYS.

United States Patent PNEUMATIC FORCE BALANCE RELAY Charles J. Swartwout,Chagrin Falls, and Earl O. Schweitzer, Wickliife, Ohio, assignors to TheSwartwout Company, Cleveland, Ohio, a corporation of Ohio ApplicationSeptember 6, 1952, Serial No. 308,826

1 Claim. (Cl. 137-84) This invention relates to electronic controlsystems and more particularly to systems adapted for use in controllingindustrial processes. Such systems are utilized for regulating aninstrumentality. or a plurality of instrumentalities in response tochanges in one or more variables; for example, for controlling a valveor the like in response to variations in liquid level in a tank or inresponse to changes in temperature or pressure. The invention isdescribed herein with particular relation to a system for controlling avalve in response to changes in a level in a tank in order to maintainthe level at a set point, but it is to be understood that the system isof general application and that our invention is not limited to anyparticular field of use.

Industrial process control systems have been known and used for manyyears. Ordinarily, such systems have been entirely pneumatic; morerecently, systems have been proposed that involve combinations ofelectric and pneumatic instrumentalities with mechanically moving partssuch as slide wires and the like in the electrical portion. Thearrangements usually are such that either time delays are encounteredbecause of the pneumaticv circuits, or

sensitive and delicate electrical components must be placed immediatelyadjacent the apparatus being controlled and frequently in situationswhere ambient con-V Accordingly, a general object of the presentinvention` is to provide a control system that is primarily electrical,

eliminates the diiculties of existing controls, provides superiorresponse and regulation, isrelatively sturdy and simple in constructionand reliable in operation. Other objects of the invention are theprovision of a control system in whichthere vare no time delays in thetransmission of a signal from one point to another so that the responseis substantially instantaneous; the provision of a system in whichmechanically moving parts and resulting friction and wear are eliminatedexcept in the primary sensing element and the tinal controlling element,and in which, if desired, friction and wear can be eliminated at thesepoints; the provision of a control system in which the response iscontinuous in al1 components, i.y e., in which there are no mechanicalor electrical devices that operate in a step by step manner; theprovision of an electrical control system in which the response iscontinuous and which embodies adjustable proportional band, adjustablereset response and adjustable rate response or any of them; theprovision of aY control system that is electric up to the nalinstrumentality to be controlled, which may be operated by pneumatic,electric or other means; the provision of an improved means ofmodulating the air pressure in the pneumatic portion of a systemembodying a pneumatic operator in proportion to the electric outputofthe control; the provision of a control system in which no delicateelectric instrumentalities such as vacuum tubes are required either atthe measuring rifice element or at the iinal control element whereambient conditions may be poor; the provision of a control system inwhich the electric instrumentalities may be disposed at any convenientcontrol point; the provision of a control system in which ordinaryvariations in line! voltage and normal variations in vacuum tubes willnot aiect the accuracy and calibration of the system; the provision of acontrolling system in which accurate controlV of the regulating valvemeans or the like in response to relatively small signals in the rangeof 0 to 1/2 volt is afforded; the provision of a control system in whichreversal of response can be effected readily and in which desiredfail-safe characteristics can be obtained with either direct or reverseresponse; the provision of an electrical control system in which themeasuring or sensing element is a low impedance device, whereas thecontrolling system itself is a high impedance device, whereby thecalibration of, the sensing element remains steady and its output is notsubstantially aiected by the operation of the control; the provision ofa control system in which means are provided for electing manual controlof the controlled instrumentality and in which the control can beswitched from manual to automatic control and vice versa without causingvany substantial fluctuation in the value of the controlled variable;the provision of a control in which the automatic control elements canbe disconnected or removed without disturbing the operation of themanual control; and the provision of a control system with a high degreeof flexibility so that it can be adapted readily to various uses andpurposes.

Referring now to the drawings,

Figure 1 is a diagram of an automatic control system embodying ourinvention and showing the application of the system for the control of avalve in lresponse to changes in the level in a tank.

Figure 2 is an enlargement of the pneumatic relay portion of Figure 1showing the details and arrangement of the valves therein.

The drawings are necessarily diagrammatic with respect to bothelectrical and mechanical components and no eiort has been made to showthe components in scale or in correct proportion to each other.

General arrangement.-As indicated diagrammatically in Figure 1, thesystem is adapted, for example, to control the level of liquid in a tankT by adjusting a valve V in conduit C leading to the tank. The sensingelement for determining the level of liquid in the tank and forinitiating the control operation includes a oat 10 which operates-adifferential transformer indicated in general at 11. The output of thedifferential transformer is compared with a set point voltage from anadjustable differential transformer 12 and the diierence in the twovoltages ampliiied by the alternating current ampliier indicated ingeneral at 14. The alternating current amplifier embodies a proportionalband control network indicated' In order to provide an adjustable rateresponse or derivative amplier, the direct current output of therectifier 16 is applied to the rate amplir'ier circuit indicated ingeneral at.17. Adjustable reset amplification, or integration, and poweramplification is obtained through the reset network and power amplifierindicated in general at 18. The output of the reset amplilier isutilized to control the electro-pneumatic power relay indicated ingeneral at 19 wherein the electrical signals resulting from changes inthe level of liquid in the tank T are converted into changes in airpressure so that the air pressure can be employed to operate the valve Vto maintain the desired level in the tank. The manual control, indicatedin-general at M in Figure l, may beiuterposed in the circuit lbetweenthe reset amplifier and the power relay. Suitable switches in the manualcontrol are utilized, as decribed below, to connect the manual control,anddisf connect the automatic control, and vice versa. 5

In practice itis preferred to construct the' Components. ofVthe-automatic control as v a compact singlelunitjhat can be replaced'simply by unplugging` one unit, and.'Y pluggingin a new unit in case offailure. This .also givesv4 flexibilityto the controlandjmak'esitpossible ,toV inter- 10 change, if desired, units having differentcombinations of' modifying controls;v for. example, aj control unitV Yomitting t he reset. amplier mightbe. substitutedjfor. al.y

complete control. All control 'adjustments areaclu'tel calibratedtofacilitate such in terc :l1 a nge of unitsTliefl manualjcontrollislalso constructed as ,a tseparatennit, so, that eitherthemanual controlunit or the automatic 'control funit" can be removedA'without Adisturbing the other. Theqentire controLj including the'automatic control ';unit` andithe manual 'control unit, can belocatedfat any 4con-- 20 venient'jcentrallocation along withithe' setpoint` voltage transformerlZ;n Only' the differentialtransformerf- 11"and the electro-pneumatic' power relay 19 are required to T', beplaced'nearthe tank T'and valve V respectivelythese elements canfbe ofsturdy construction'and able to jwith- .25 standgpoor operationconditions. i

`Power snpply'z-El'ectric power for the automatic' control-is suppliedby transformer 21, the primaryj of which is'preferably'` connected tothe output of l'a conventional. constant'voltage transformer 20 that isenergized from an 30 ordinaryj4 alternating current source; thesecondaries ".of transformer 21 are used to supply'the variousvoltages.' required'by the circuit components. The.. B1!- voltageis'supplied by a full wave bridge rectifier indicated in' general at 2 2,therectifier-being energized by. secondary 35 23'and'including a lter,condenser 24.`

Inpzft rcintura-AsV noted' above, the control operates.; Yuponthes-basis of "a comparison between a voltage that. varies with thevariable to be controlled, and anadjustable set point-voltage. In-theembodiment shown these .volt-` 40 ages V are'derived from' substantiallyidentical dilerential; transformers lland 12. `It is, to be understoodthat if it' isdesiredto-makethe'control responsive to morethanone-variablc, a'correspondin'g number of transformers 11; each actuatedby an appropriate sensing element and".45.

withV the outputsofthe transformers combined,.jmay be employedi-Differential ltransformer 11' comprises apri- A mary-25, -secondariesj2,61and 27gand a ferro-magnetic, armature V28thatiSv-connected by asuitable mechanical;A connection-29"with'ithe Viloat .10 in tank T.' Theprimary 50 25f1is energizedgby the v constant voltage transformerj20fThe secondaries 26"and"27 are connected tobuck'eac [jother-withltheresult that ,when the armature 28`is'in'mi'd-i'positionequallvoltages are induced in SeCOndarieSJG'and' jZTfand-'theontputof thetransformer 11'is zero. VAs .the 55' armatureisjdisplaced-in Veither' direction fromthe mid-" positiondhe-voltagesinduced'invthe secondaries'are unbalanced; the'unbalance-issubstantially linearly proportionalto --the-displacementY of thearmature so that the output-voltage varies with the displacement ofthe'arma- 60 ture:-V Displacement of` the armature. in one directionfrom the mid-position resultsin `voltages that .are 180"out-of-'plasewith'voltages inducedgwhen the, armature isvdisplaced-'inthe'opposite'direction from mid-position; howeVer,-theapparatus preferably is arranged 'so' that '65 within' the normaloperating rangethe displacements of the armature-areonthe same sideofthe zero or mid-position:

Inorder-to=make -it possible toadjust'the output-volt-Y age.-calibration ofthe-.differential transformer 11, the--primaryfsupply'circuit includes a variable-resistor- 32"and '-70 acondenser-33?which-prevents the :phase of the output-d voltage `fromshifting when the.output voltage -isadjusted by-m'eans'- of fresistor132.' The circuit of the primary-25" also `includes conventionalvrtiXed- VCalibrating :resistors -34 and 352'and vanegativetemperature-coecientresistor- 36#- f "isomers 4 '1, in shunt withresistor 35. These resistors provide a temperaturecompensated circuitthat can be adjusted'to make the output of transformer 11 substantiallyindependent of changes in ambient temperature within reasonable ranges.In the preferred assembly of differential transformer 11, the negativetemperature coeicient resistor 36 along with Calibrating resistors 35.and;34 are mounted immediately adjacent the windings v25, 26--and 27-of thetransformer so as to be subject to the same ambient temper- Thesewindings. andA resistors 34, 35 and ature changes. 36 preferably aremounted'in ahousing or enclosuremade of suitable f magnetic materialand'- indicated 'in general by dotted lines at E in the -drawingsnThe;.housing E acts as a magnetic shield for the transformer windingsand in .addition insuresthatfthe-negative temperature coefcient resistor36 and the windings will be subject to the same ambient temperaturesthereby enhancing the accuracy ofthe transformer.

The.rsetpointlvoltage. is Aderivedlfrlorn .transformer v12V WhCh. is.substantially identical with transformer .1!1", 'the i primary. off the'transformen also bein'g'energized from constant voltagetransformer20`through`calibrating and 1 v temperaturev compensating circuitsidentical 'with those described',inconnection with transformer 11. InAtransformer 12, however, theA Var-maturej37.isfmanually adjustable to.give the desiredV output by means of^knob38 which isrsupported forrotationb'ut held against longitudinali Vmovement iu a panel '39.''I'hethreaded 'end 40"ofI the shank; of the knob engageswithin,athreadedaperture in v1n1t-,41;whichV is. restrained againstrotation .and is operatively connectedto armature 37. Thus', rotation ofthe knob 38 results-in longitudinal movementof. the armature 37 andadjustment of .the output voltage of the v transformer 12'. In practice,the control knob may be calibrated accurately. interms `of the variableto be controlled; i. e.theliquid"level 'in tankT in the. presentexample; manually adjustable, itl is contemplated that theadjustments'maybe' made automatically, for example, according ,to apredeterminedV time schedule; vor the` set point voltagernay. becontrolled in response to changesin one or more other variables, thusmaking the control responsive Eto a relationship between a plurality.of,variable's..

The signal and set point voltage outputs of transformers 11a`nd'12,"respectively, are Vofthe same phase and 'the same order ofmagnitude, bothvoltages leading the powernhodigmentjioflthe apparatus.Th'etsignal and set point voltgesfconstitute the inpntvoltags of thecontrolci'rresistorsV 43and 44 are used-to attenuate ,slightly thefsetpoitvvoltageibefreit is fedqto'the cathode of the tube 42`;"jjthe'attenuationis' such" that the inputgainsof the tubefor'signals'V 'ofequal'strength'fed'into the cathode and .grid `circuits are equal.,`Hence, .tube 42"becomes .a

true.di1erential amplifier that ispsensitiveonly to the dilerence'Lbetween the signallinput voltageand the set. pointlinput voltage-and' isindependent of the actualdevels` of these'signals. This`difr'erence'then becomes' the control'signalvoltage that `isamplied'and modedlby, ther control circuits and utilized to operate thepower. relay 19. The output of the differential,v amplifier tube 42'iseither in phase with the, signal and set'point-'input voltages orl80`out of phase therewith, dependingupon whether the signal voltageexceedsior is less than the' set point-voltage.

The .magnitude offthe. output rvoltage depends o n the amount'ofdifference betweenthe signal 'and set` point voltages:`

Alternatingecurrenteamplfier and" proportionalL bandvcontrol.'-'I`l1'eA;-C. 'amplien--indiatedin general at 14;- isessentially la-conventonal three= stage;resistance-capaciiz 5tance--coupled-'ffead@back `:ampliti'ergrthe-tubes@.:4'1L2and 448.:-

Whiletheset'point voltage is shownas being asoosis furnishing stages ofamplification in addition to the amplication of tube 42. Resistor 49 andcondenser 50 are utilized to reduce the amount of alternating currentpresent in the voltages supplied to the plates of the amplifier tubes 42and 47. Resistor 51 and condenser 52 provide decoupling between the rsttwo amplilier stages. In order to eliminate high frequency componentsand oscillations in the amplifier, resistors 53 and 54 and condensers 55and 56 are employed. Resistor 53 and condenser 55 also function tointroduce a lagging phase shift in the signal voltage and thuscompensate for the leading signal voltage output of the amplifierresulting from the 58 leading signal and set point inputs and bring thesignal voltage into phase with the power supply.

To summarize, the signal input, which varies with the level of theliquid in the tank T, and the set point input are compared in thealternating current amplifier and the dierence between the signal andset point inputs are amplified. If there is no diierence between thesignal input and the set point input then the control signal is zero andthe output of the amplifier is also zero. Depending upon whether thesignal input voltage is greater than or less than the set point inputvoltage the amplitier delivers an output voltage that is in phase withor 180 out of phase with the power supply, the amplified control signalvoltage being proportional to the dierence between the signal inputvoltage and the set point input voltage.

The action of the alternating current amplifier is modilied by theproportional band control, which is, in effect, a sensitivity controlthat determines the amount of the respouse of the controlled element togiven changes in the variable being controlled; i. e., in the presentinstance, the change in the position of the valve V in terms ofpercentage of its full travel that takes place in response to a givenchange in the level in the tank T in terms of percentage of the fullscale movement of the diiferential transformer 11. This is defined asthe percentage of the full range of the input signal that is required toproduce full travel of the valve. At 100% proportional band the valve ismoved through its entire range of movement in response to a change inthe level of the tank equivalent to the full range of the levelmeasuring element. At proportional band only a 5% change in the level ofthe tank produces a full stroke of the Valve. Thus low percentage ofproportional band means high sensitivity, high percentage means lowsensitivity. In the present system proportional band control is obtainedthrough the network which comprises a negative feed back circuit leadingfrom the output of the alternating current amplifier at point 57 and tothe input of the amplier through condenser 58, adjustable resistor 59and resistors 60 and 61. Resistor 59 is accurately calibrated so thatits adjustment can be readily determined to facilitate interchange ofcontroller when desired or necessary. Adjustment of the resistor 59governs the amount of negative feed back and in a preferred embodimentof the system makes possible control of the proportional band from 3% to200%. Also, the condenser 58 and resistor 59 constitute a phase shiftnetwork that advances the phase of the feed back signal approximately 58to match the input voltages derived from transformers 11 and 12.

By this means adjustable proportional band control is introduced intothe system simply and with a minimum of components. The use of negativefeed back to accomplish this insures stable operation of the ampliier.

Phase-sensitive rectijer.-The phase-sensitive rectifier takes thealternating current control signal output of the alternating currentamplitier 14 and converts it into a direct current control signalvoltage that is proportional to the alternating current control signalvoltage, the direct current voltage being positive when the alternatingcurrent output of the alternating current ampliier is in phase with thepower supply and negative when this output is 180 out of phase with thepower supply. Thus the phaseJ sensitive rectifier produces a directcurrent control signal voltage that is proportional to the differencebetween the input signal voltage and the set point voltage and hence isproportional to the level in the tank T as compared to the desiredlevel.

This is accomplished preferably by the circuits shown in which areference voltage derived from secondary 62 of the transformer 21 issupplied through resistor 63 and condenser 64 to the center pointbetween rectiliers 65 and 66. Condenser 64, rectifier 65, resistors 68and 69 constitute a shunt rectifier circuit which produces a directcurrent voltage of the polarity indicated between the point 71andground. By increasing the resistance of variable resistor 68, theeffectiveness of rectifier 65 can be reduced which makes it possible toadjust the value of the voltage between point 71 and ground by means ofvariable resistor 68. L

Condensers 72 and 64, rectier 66 and resistor 73 also constitute arectifier circuit. Assuming first that there is no output at theterminal 57 of the alternating current amplifier, then the referencevoltage will produce a voltage between point 71 and across resistor 73of the polarity indicated. Next, resistor 68 is adjusted to make thevoltage across resistor 69 and the voltage across resistor 73 equal withno output from the alternating current ampliiier, so that these voltagesbalance and there is no voltagebetween point 70 and ground. With thecircuit so adjusted, the application of an input signal in phase withthe reference voltage will reduce the effective alternating currentapplied to the rectifier circuit involving rectifier 66 causing thevoltage across resistor 73 to be lower, while if the alternating currentis out of phase with the reference voltage the voltage across resistor73 will be increased. The amount of the reduction or increase dependsupon the amplitude of the applied alternating current voltage. inasmuchas the reference voltage is applied to both rectier networks, theoutputs of which are balanced against each other, while the signalvoltage is applied to only one network, variations in the referencevoltage amplitude within reasonably wide limits are ineffective tochange the output whereas variations in signal voltage input result insubstantially linear variations in output. The reference voltage must begreater than the maximum signal voltage, and in practice the referencevoltage is normally three to four times the maximum signal voltage inamplitude. Resistor 74 and condenser 75 are added to the circuit tofilter alternating current ripple from the output of the rectifier.

The response of the control can be reversed, that is, the output can bemade to Vary directly or inversely with the signal input voltage bymerely reversing the connections to transformer secondary 62 and thusreversing the phase of the reference voltage supplied to the rectifier.

The rectifier produces a direct current control signal voltage that isproportional to the difference between the signal input voltage and theset point input voltage applied to the alternating current amplifier;this direct current voltage after amplification by the rate amplier andthe reset circuit and power amplifier is utilized to effect theautomatic control of the valve V.

Rate amplz'jer.-'Ihe direct current control signal output of thephase-sensitive rectier, which is proportional to the difference betweenthe signal input and the set point input, is fed to the direct currentrate ampliier indicated in general at 17. Essentially this comprises adirect current amplifier with a gain of, for example, about 50 and witha negative feed back circuit arranged with a time delay network. Thefeed back reduces the normally high amplification of the amplifier tosubstantially unity for the steady state; the time delay network reducesfeed back for rapid changes. When no rate action is employed, the outputof the amplier is substantially equal to the input regardless of rate ofchange. When rate action is employed, a sudden change inconfar-ad'condenserhasbeen f employed 5 at thispoint.

Y4" trolvoltageeinput toethe Lrateamplifier -'results-` in "ihghampliticatiom'-initia'llyv because thered is. the'feem back* delay;vHowever VAafter a5 time interval@ determined by th-adjustrnentfofa-resistor-'capacitor network; the feed backV again-makestheeoutputsubstantiallylequzrlto-fthei 5l input. The direct currentoutput of theirate arnplijerV thus-fisnot)-only'proportionaletoi tle`-deviation between v the' input'- signal-f voltage lande-thzeeset pointvoltage '-bnt alsoto f the 'rate loflf'ch'ange/l of the-impidesignalwlith tree 'f spectto 'the' Lsetepointx voltage.1w I-Ieinc'e,$4the effectief-the- 10 rate amplier'isto-'make theecontroller-sensitiveto rapid changesinJ 'liquid-ilevelf'in tankT'fand provide correctivecontrollin proportion-toithe rate-'ofi hang'evoffilevels Y Peferabl-y,-fth'ese C'results are raccomplished'b'y Efeeding theAAoutput'ofthe-phasesensitivereetier tol'th'e-'gr-id"'of-b l5 tube 77throughresistorA 7S, lcapacitor 79'being-- employed to i it'er 'outihi'gh-frequencye componentes The negative feed back circuitincludesriixed resistors 81 and-82,'f-varable resistorSdf-'andfcondenser :85. In order to give the requirede-'time=delayand=accuracy/,--conde'nser- 85 should 20 bel-a high-quality,-V low-le'ak-age condenser-.ot substantial capacity,; for example,l inoneEembodiinenta` six micro- Resistor- 845 preferably- -is astep-'type-resistor-as shown-andis'accurately'- calibratedtopermitconvenient interchange 25y change="-tl1`e1total -resistance -inythe-'grid circuit in any35`VY way-but simply changes the-amount:of-resistance` that is in serie-s-with-condenser'SS.Thus-for-steadystate -conditions-thelfee'd'lbackfis Vthe same regardlessof the point 1 of adjustment ofthe resistor-84.

When-it is-desir'ed---to eliminatecompletely the rate 40 eifect;the-adjustablecontact- SG--is moved out of'-contact-withiresistor--iand-toa position -where thel condenser-SS'is inseries `with resistor 83 and disconnected from-resistor S4.- Thiscircuit simply places a chargel on-condenser- SS-.soY-that when therateaction -is lturnedY "j on-againthere-will be -noundesiredfluctuations ofthe control.-

In'order-to'flower tho plate -voltage to a value-'suitable for feedingback -to-the-v gridof-the tube, a Iioating power supply-Inuit:-comprising--rectiers S75 and 88, capacitors 89, 91T-and 91-vandpresistors 92, 95 and 94 is interposed in the feed-back circuit.VThis-unit receives its-power from secondary-95 Vof' the -.powertransformer 21-` and-- constitutesa -rnore orless conventional voltageVdoubler circuit. The values of grid-resistors 78v and 83 are very highcompared to the value-of plate resistor- 96-with the result-that thereis no-appreciable-current ow in the gridcircuit and through the floatingpowerk supply.

Thefrate amplifiercan be adjusted from :a condition in which;the .rateaction is completely eliminated to a con dition in which a signalimpressed on the system'as aresultof asudden-change may begreatly-,ampl-iied to give` prompt and feifectivelresponse -to a quickchange and nonnalxresponsento slowenchanges -in input.

`Resetrcrcuztand.power. ampliye1nThereset circuit -Ca and;power;amplierrgivesan output proportional to the inputrand; theA time'integralY of the-input. By kmeansof this ipartv of the-controller', avery small input signal, if it'persists fora suii'cient Yperiod oftime,can be made to give'the maximum output-signal that is-obtainable vwith fO the-system. This-action enables; the controller-to compensatenforHwidechanges in load conditions without sustained deviations of thecontrolledvariablefrom the settipoint:`

AInthe embodiment-shownythe1 powerampliii'ei"inclu'desT 75 geneigte-1avoltage amplieretubeeltlthatfurnishesamplidivolt e ages in proportionLtoethe' deviation ofi'theeinputj-lvoltageefromvthe-setepointvoltageand:alcathodefollwer power amplifiretube 101? A`signal impressedfontthe cathode' ofi-tube: 100 isf amplified thereby 'eand appearsV at point' 102, the-outputofthe fll'oweretubelillf? ByDcombiningr a` negative feedlibackL circuit` with "apositiveeedbackcircuit; the `overalli gain ofitliese:amplifiers` is #caused 'tober verysmall;V practically unity; for rapid' changes of signal input; whilethe-gainisecause'd :tcr'lbecome Very--largej'r ofthe order.v of2001101'"-moreeto-*l"frsteadyP orvery slowA changinglsignalL inputs."The;"negativenfeedl back" circuit-connects to' poi'rxt"-102^,l whichxistlie-outputtern'iinarl f' of the reset circuit and -thefentirecontrollerunit;con#

` tinues through the L`floating#*powerl supply indicatedJ in `fierandi-is"energizedifrbmJ theLsecond-ary -106-1ofthefpower ftransformer2.1:-` It-is--inserted intoethecircuit-'for drop'- pinglthee-output `'voltage down'toa-valuefsuitable for feeding into Mthe gridof tube 100. Switch 107 ,when-inf the-posit-ionhshownshorts.cute-condenser 104, provides direct current negative-feedback-lto :tubev100and reducesf the -gain to -unity '1 andi-thus l eliminates i thefunc-:tion offthe l reset network-'.1- Wi-thswitohi107-openg directcurrentfeedrl bacl ;-isf-bllockedLv v.byl condenser .1049 and lnegati-veIA feed- I backccurslonlyfor@ rapidly changing--signalsgL.Theupositivelfeedlt-baclcf-circuit {also-connects Ito `--point- 102 onthe l:cathode-of -follo'werE tube-1013i passes throughpowerrsupp'ly..103',- and-lthe'n---'connects-"folthei cathode of4 tubeM01-throughlpositivefeed- Shackresistors 1108,# 109iThisxisregenerativefeed back .andlhas-the eiect-of- .greatlyj increasingthefgain :of-.'.the uamplier-ffor Veven -verysmallz input;.si'gnals,ihence; :the :gai-nv` off theamplifierA -in effect Vrent Ynegative :feed.backlisL blockedtby:condenser: 104,--V

and.. the amplifier"reactssolelye'toV positiveLfeedLback. However, when.the input sip-mals v'changelrapidlyin value and have highlfrequencies,the gain o-fzth'e amplifier ap#A adjusting the value.;4 ofV resistorfql;from YYminimum to'v 'Ymaximunm the reset rateina preferred embodiment'is `adjustable from a maximumgofi 20V 4repeatslfper: minute .to-1

a -minimumbf .03 repeatsiper-,minutes.l

The switch,Y `10.7-.is.used: to'vshunt; out `condenser V104 when it isydesiredztoeliminateureset action. Relay110..

when, energized -fshuntsi resistor 105 rwhen. reset "switchr, 107is-closedandeis utilizedas.appears more .fully below,

to provide fast-resetv action'whenzthe Vmanual control` unit is in-themanual-balance position withV the result'y thatL there is nosubstantialcuctuation-in -thecontrolledvari'- able when the .control isswitchedtfrom manual to automatic operation. Y Y

With this.. arrangement Vthe power amplifier provides the power requiredfor operating the controlled instrumentality such` as thepower VrelayV19 and the reset action gives full travel to lthe valve'V'ina'prede'termined period ottime vregardless of how small'thedeviation' ofthe 'input 'voltage' from the set Vp ointvoltageV may be.'

While the lalternating'currentamplier '14, rate'amplifier-17 andV'poweramplifier of Athe reset circuit 18are describedv above' ashavingA a totali-of*V six vacuum tubes, `it` vwill bef-'understoodthe^"elements'of "these tubes may be* 9 combined in pairs in the sameglass envelopes so that the physical number of the tubes in thecontroller may be reduced to three, that is, three duo-triodes.

Electro-pneumatic relay.-As stated above the output of the automaticcontrol unit isa direct current voltage having a magnitude that is afunction of the deviation or error in liquid level in tank T as measuredby the oat 10. This direct current output is used to control the actionof valve V in a manner to correct the deviation or error. In order toconvert the electrical energy of the output of the control unit into aforce capable of accurately positioning the valve V, theelectro-pneumatic power relay 19 illustrated schematically in Figure 1is provided.

The electrical components of the relay 19 comprise a moving coil 115disposed in the eld of a permanent magnet 116 and mechanically connectedto a lever arm 117 adapted to pivot about fulcrum 11S. Fulcrum 113preferably is a spring pivot to eliminate friction and backlash. Whenthe relay 19 is automatically controlled by the automatic control unitdescribed above, coil 115 is energized by the output of the automaticcontrol unit and is connected thereto by means of leads 119, the manualcontrol unit M and leads 163. The force which coil 115 exerts on leverarm 117 as determined by the output control voltage of the controlsystem is normally balanced by a force exerted by the pneumatic pilotvalve assembly indicated in general at 120. When a change in the outputof the controller occurs, the force exerted by coil 115 on lever 117increases or decreases thereby momentarily upsetting the balance offorces acting on opposite ends of the lever. As a result of thisunbalance, the lever 117 pivots slightly about fulcrum 118 therebycausing the end 121 of the lever to move up or down which initiates achange in air pressure in the assembly and restores the balance offorces. Variations in voltage result in changes in or unbalancing of theforces exerted by the pilot diaphragm 125 and coil 115.

The pilot valve assembly 120 controls the supply of fluid, that is, air,under pressure to a conventional pneumatically operated diaphragmactuated regulating valve V in accordance with movement of lever arm 117and comprises a split housing 123, the upper part 1235i having an upperchamber closed at the top by a diaphragm 125. A hollow sleeve 126 issecured to and movable with diaphragm 125 and is also connected to andmovable with the end 121 of the lever arm 117. The lower end of sleeve126 constitutes a valve seat which is normally engaged and closed byvalve head 127.

Air under pressure is carried by conduit 129 to chamber 130 within thehousing 123. Communication between chambers 130 and 124 is made throughport 131 which is normally closed by valve head 132 connected by valvestem 133 to the upper valve head 127. Downward movement of diaphragm 125causes valve head 127, stem 133 and valve head 132 to move downwardlycorrespondingly. The pressure of air in the chamber 13@ acting againstthe bottom of valve head 132 urges both valves 132 and 127 upwardly to anormally closed position.

VUpper chamber 124 of the housing 123 communicates via passageway 135and conduit 136 to the underside of ilexible diaphragm 137 secured toand located in the lower part 1231; of the housing 123. Thus, the unitpressure on the undersides of diaphragms 137 and 125 are substantiallyequal. The effective area of diaphragm 137 preferably is substantiallygreater than the area of diaphragm 125 and hence the force exerted onthe former is proportionately greater than that exerted on the latterfor a given pressure on the diaphragms. Another exible diaphragm 138 issecured to the lower part of the housing in vertically spaced relationto diaphragm 137 and defines the bottom of another pressure chamber 139in the housing.

The diaphragm 138 has a hollow sleeve 140 which has a larger diameterthan sleeve 126 on diaphragm 125 and which is connected by member 141 toand is movable with lower diaphragm 137. Valve head 142 seats on the topof and normally closes sleeve 140 and 4is connected by' stem 143 tovalve head 144 which seats on and normally closes port 145 betweenchamber 139 and supply air chamber 130. The area of the opening of port145 preferably is substantially greater than the area of the opening ofport 131 and hence the air handling capacity of the former iscorrespondingly greater than that of the latter. The space 146 betweendiaphragms 137 and 138 is connected to atmosphere by conduit 147.Chamber 139 is connected by conduit 148 to a conventional diaphragmactuated regulating valve V, the pressure of air against the diaphragmbeing balanced by a spring 149 that in the embodiment shown in thedrawing always tends to open the valve V.

The housing 123 preferably is constructed in two parts or sectionssecured together along the parting line Z at about the central sectionof the housing. This construction permits convenient access to thevalves 132 and 144 for inspection, cleaning and/or replacement whendesired k or necessary.

The operation of the electro-pneumatic relay is as follows: Assume thatthe output of the automatic control unit, for example, calls for adecrease in supply of liquid to the tank T. The output current from theautomatic control unit for such a condition causes relay coil to moveupwardly as viewed which in turn causes lever 117 to pivot clockwiseabout its fulcrum 118 and the opposite end 121 of the lever to movedownwardly. This movement of lever arm 117 presses diaphragm 125downwardly and causes valve head 132 to open admitting air underpressure to chamber 124 until the pressure in chamber 124 builds upsufficiently to cause diaphragm 125 to exert an equalizing force againstlever arm 117 at which time a state of equilibrium is reached with theforce exerted by the diaphragm balancing the force exerted by coil 115and valve head 132 closes port 131.

The air pressure in upper chamber 124 is commuicated to the underside ofdiaphragm 137 by passageway 135 and conduit 136 so that diaphragm 137moves upwardly in response to the increase in pressure. This motion ofdiaphragm 137 is transmitted by member 141 to diaphragm 138, stem 143and valve head 144, the latter thereby opening suiciently to admitsupply air under pressure through port to chamber 139. When the airpressure in chamber 139 reaches a value such that the force on diaphragm138 will be equal and opposite to force on diaphragm 137, a state ofequilibrium is reached and valve head 144 closes. The increase in airpressure in chamber 139 is transmitted by conduit 148 to the diaphragmof valve V which causes the valve to close and reduce the flowfof waterto the tank.

When the opposite condition is sensed by the primary i measuringelement, that is, when the water level in the tank decreases, the directcurrent output of the automatic control unit is reduced, the coil 115exerts less force than the force exerted by diaphragm 125 and acondition of unbalance exists whereby lever 117 pivots in acounterclockwise direction about fulcrum 11S and the end 121 of lever117, diaphragm 125 and sleeve 121 move upwardly. This movement of sleeve126 unseats valve head 127 from the bottom of the sleeve and permits theair under pressure in chamber 124 to exhaust through the sleeve toatmosphere, decreasing the pressure in chamber 124 and on the undersidelof diaphragm 137 which unbalances the opposed forces exerted by thediaphragms 137 and 138.

Tlif fis 'to Y say; diaphragmflsiwill* exert la' lesserKforce* by reasonof the pressure decrease than the force exerted byVV diaphragm 138i As--'a` result," tliese'diphragms move downwardly, the downward vlmovementofl'diaphragm' i138 'i causing unseatin'g of valve' head-142'"from"sleeve140liand 5YV everg-the lcorrespondingly*small openingsinsleeve`126" and port 131- on Vwhich lva'lves 127 vand--132,respectively;A

permittinge'ch'amber 139- to exhaust to' atmosphere throughconduit-147'. This exhausting ofchamber Y139'e:luc `e`s the" pressurez'of air therein fwhich in turn reduces the" pressure' acting'. on thediaphragm lof' valvefV and *permitsf valve' spring 149 to 'open' thevalveargreaterlamount and increaseH the owf'of water to the tank TLThe-action ofthe` electro-pneumatic relay* 19' -may `be Vmade' tocorrespond Seither-'directly 'or inversely to the -olt-I put ofthe'automatic controll'erfby'reversingitheconnection3 rectifier, permitsthe whole Lcontrol sy'ste'r'nto'beY 'made' fail-safe regardlessoffthe'ttypeof regulatingvalve em`` ployed. Y i

Zeroadju'stmentof the-relay 19,` that is, adjustment of the `Vcontrolair pressure yinthe pilot'valve assembly to a Vdesired'value for a givenoutput of the controller, preferably is accomplished by means' ofspring151at se'a't,arel capable of controlling for-handling only;small-- quantities of air. Inl order that the` power vrelay will becapabler of 'quickly actuating, for example,` a relativelyllargeregulating1valve'V, a secondlorbooster stage'fisv provided in therelay, which boosterstage includes a supply port 145': and an exhaustsleeve 140, each having;VV relatively large openings compared toport 131andsleevel' 126 and thus having substantially larger air handling-iY V15capacities forfproperlyI actuatingcvalve V. The capacity of thepilotvalve assembly 12,0 may also lie-increased by providing adiaphragmf138 havingv an effective areaV47 smaller than that ofdiaphragm 137 so that a corre'-- Yspondingly greater control pressure in`chamber 139 is 'required to elfe-ct a balance Vof forces exerted bydiatached=at one end to lever 117 asshownand'lwhose tension yorcompression is adjustable Yby means ofV a nut'V and b'oltV assembly 152which securesV the other. end of the Vspring to an anchor 153. The eiectof spring l151'ist0 increase or decreasetherforce exerted by the coil151 on the lever. v The total force exerted by the coil and spring 1sadjustable by means of the nut and bolt assembly 152 v and hence thecontrol pressure in pilot valve assembly 120 and consequently the degreeof Vopeningof valve V Y given output voltage Y is adjustable to adesired value for a of the -controller;

Adjustment of 'therange `or spanNof opening of their. valve V for agiven` range of output 'of`thecontrolleis' accomplished by meanskofvariable resistor'154which shunts vacross relay coil 115 and permitsmanual variation `of 'the' percentage of control loutp'u't currentpassing Vvariation' of outputvcurrent ofthel controller-is'llmilliamperesand.- for such'a'variation.` of current-passing throughcoil-115 the range of opening of valve l1V is maximum or 100% of itsfull travel.- With resistorfl adjusted-'sothathalf the controller outputcurrent passesf through the coil-115, that is, the variation of currentflowing through'coilA 115 is 2 milliamperestherange -ol opening of valveV is vreduced to for theY full range of controller output. i isconnected in seriesv with coil 115"and'resistor 154`in order to maintaina constant resistance load on the controller regardless of the'settingof shunt resistor 154 and An additional variablerresistor 155n VGO thusto maintain the impedance of the circuit at the'desired level. Y

The relay Y19 is a force balance mechanism, that is, a

force' proportional to the output voltage of the automatic control unitis balanced against an airpressure sothat theA air pressure suppliedtothe regulating valve V'bearsl a proportional linear relationship 5toVthe output voltage from.the control unit.Y The two stage diaphragmarrangement. of the relay has tlieadvantage; of 'increasing theairhandling capacity ofthe device withoutdecreasing' its sensitivityy torelatively small signals from-the automatic control unit. That isto-fsayfthegarea'of diaphragm. 125 Yshould lbe relatively small so'thatthe balancingy forces Vwhich the diaphragm exerts `will besubstantiallyl` in the vsame order of magnitudeas the forcesH'D` tiveufeedback 'controll th'at`mak`e'sit possible tobbtanm phragms 137 and138. In addition, the pilot valve assembly is completely' enclosedYthereby protecting the working' parts 'from' dirt' andV other foreignmatter and' insuring trouble free" service.

asV to givelthev desired result in the-'controlled instrumen'-`tality-for a change in theevariable being controlled. InA :the presentvexample'these connections are arranged'iso Vv that as the level ofliquid in tank T falls the'pressureof the'air supplied to valve V isYreduced thus permitting..

spring 14S tofopen the valve and supply more'liquid to the tank to bringthe level back to the set point.

3i Yopposite action takes place when the level rises;

Other preliminary adjustments include the adjustmentv ofthe tension ofspring 151 to setY the Vzero position of valve V, and the adjustment ofresistor 154 to adjust the .range of movement of valve V for a givenchange in the Acurrent owing through the coil of the power 4relay. Also,the set point transformer 12 may be calibrated in termsof theilevel ofliquid in! thev tank T.

When power is supplied to the system'it is only neces'- Vsary toadju'stthev set point transformer 12 to the desired' ovvalue andthecontrolaautomatically operates the valve V' to maintain the levellinthe tank VT as near to the set point as possible.- The level in the tankVT is'measuredfby the diterential transformer 11. Theoutputs 'of-dif#ferential transformers 11 and 12 are compared 'inl-tube" 42 andthedifference, if any, amplified by the alternating-.r

current amplifier made up of tubes 42, 47 and 48.` Proportional bandcontrol is providedy by the adjustable resistor 59 which controlsfthenegative` feed back tofthe* s grid of tube 42. The width ofproportional-band selected l depends upon the nature of the variablebeing controlled,v

for example, if the level in the tank T'uctua'tes only slowly a wideproportional band giving'relatively slow response and great stabilitymay be employed. If the A`changes in the level in the tank are morerapid, the 'resisduce Va greater change in the opening' of valve-V.

The' output signal of the alternating current ampliier,

5 and set point input voltagesaas 'modiedb'y the proportional',bandi-control, is rectiedvby the phase-sensitive rectifier. inasmuch asthe rectilier involves'the'com-l parison of the output signal with -a'reference voltage derivedffromfv the same alternating-current source,that is, the power supply forthe entiresysternchanges inthe voltage oflthepower supply vwithin reasonable limits do not aect the output of therectifier. VThe direct current signal output yof 'the phase-sensitiveYrectifier is 'applied to the `rateamplifierwhich embodiesn"adjus'tble-nega` Smmry 0f`0peratz'0n. -In placing the apparatusincondition for automatic operation, the",connectonsto` transformersecondary 62 and coil 115 are'arranged so` TheVV tor 59 maybe adjustedto give a narrower proportional* band so'that a given change in level intank'T will pro-A Y which is Ya function of the yditlererlcebetwe'enthesignal a large amplification of the signal for rapid changes while thereis substantially no amplification for steady-state or slowly changingsignals. This gives stability of operation when that is required andrapid response when changes in the level of the tank T demand rapidchanges in the valve V in order to prevent wide departure of the levelin the tank from the set point.

The signal voltage, as modified by the rate amplifier, is next suppliedto the power amplifier and reset network. In this section of thecontrol, the amplification is through a voltage amplifier 100 and thecathode follower 101 with positive and negative feed back from thecathode follower to the voltage amplifier. Condenser` 104 in thenegative feed back circuit eliminates direct current negative feed backwhile adjustable resistor 105 leading to ground provides, with condenser104, an adjustable time constant in the negative feed back. Thesecircuits give adjustable reset action or integration to the control sothat the valve V can be moved to the limit of its travel in apredetermined period of time regardless of how small the signal input tothe cathode of the voltage amplifier tube 100 may be. This makes itpossible for the valve V to compensate for wide variations in theamounts of uid required to maintain the level in tank T withoutrequiring any substantial difference between the level in the tank andthe set point. The negative feed back circuit provides stability ofoperation under changing conditions and makes the reset circuitresponsive only to steady-state or slowly changing conditions.

The output of the power amplier is supplied to the power relay ortransducer 19 which accurately controls the pressure of air supplied tovalve V in response to changes in the flow of current through the movingcoil 115.

From the foregoing description it will be evident that the inventionprovides a continuously variable electrical control system whereby oneor more variables may be utilized to control one or moreinstrumentalities. The control acts substantially instantaneouslyinasmuch as it does not require long pneumatic lines. All delicatecomponents of the system can be placed in a central location away fromthe severe operating conditions frequently encountered where suchcontrol systems are needed. The control is compact and flexible and canbe adapted to many uses and purposes inasmuch as it involves all of thefactors required for rapid and accurate response Iand at the same timestable and reliable operation. The control involves no moving partsexcept at the sensing element and the power relay; hence there are noproblems of friction, backlash and wear. The components may be standardelectrical components of known reliability. Only three double triodetubes are required to carry out all of the control functions and othercomponents are kept at a minimum so that the apparatus is compact andcan be constructed at reasonable cost.

Those skilled in the art will appreciate that Various changes andmodifications can be made in the preferred form of control system`described in detail herein by way of example without departing from thespirit and teachings of the invention. The scope of the invention isdefined in the appended claim.

We claim:

In a system of the type described having 'a control unit, a power relaycomprising means for exerting a force proportional to the output of saidcontrol unit, a. pilot valve assembly adapted to balance said force andto transmit a control pressure proportional to said balancing force,said assembly comprising a split housing having two parts detachablyconnected together, a first pressure chamber in one housing part, acontrol pressure chamber in the other housing part, a supply pressurechamber adjacent said first and control pressure chambers, and means formaintaining a pressure in said first and control chambers proportionalto the force exerted by said first named means comprising two portsconnecting said supply chamber to said first pressure c-hamber and saidcontrol pressure chamber, respectively, valves associated with saidports for controlling the flow of fluid therethrough in response tovariations in the force exerted by said first named means, said portsand said valves being located adjacent the parting line between said twoparts of said housing whereby upon separation of said two housing partsto be accessible for inspection and maintenance.

References Cited in the tile of this patent UNITED STATES PATENTS2,219,062 Almond et al. Oct. 22, 1940 2,285,540 Stein et al. June 9,1942 2,310,298 Kuhl et al Feb. 9, 1943 2,336,887 Piron Dec. 14, 19432,461,026 Bilyeu Feb. 8, 1949 2,517,051 Swenson Aug. 1, 1950 2,550,666Bilyeu May 1, 1951 2,582,088 Walthers Ian. 8, 1952 2,688,253 MarksonSept. 7, 1954

